1. Field of the Invention
The present invention relates to a switching power supply apparatus using a ringing choke converter (RCC) system.
2. Description of the Related Art
Conventionally, a self-excited RCC (ringing choke converter) serving as a self-excited converter apparatus has been widely employed as a switching power supply apparatus for low power due to the simple circuit configuration thereof. FIG. 6 shows a block diagram of a typical self-excited RCC type switching power supply apparatus. FIGS. 7A and 7B show waveforms of primary and secondary currents of an output transformer. The operation of the conventional switching power supply apparatus will be explained briefly with reference to FIGS. 6 and 7A, 7B.
When the switching power supply apparatus is supplied with AC power, an oscillation field effect transistor (FET) 5 is turned on by a starting resistor 6, so that primary current flows through the primary winding N1 of an output transformer 29 and increases linearly (Ton period in FIG. 7A).
Current is induced in a feedback winding NB due to the primary current, and the induced current is supplied to a charging capacitor 14 provided at a base terminal of a switching transistor 13 through a charging impedance formed by a resistor 18, diode 15, resistor 16 and zener diode 17 thereby to charge the charging capacitor. Then, the voltage of the base terminal of the switching transistor is increased to turn on the switching transistor 13. When the switching transistor 13 is turned on, the voltage of the collector terminal thereof becomes substantially zero, and so the oscillation FET 5 connected to the collector terminal is turned off. When the oscillation FET 5 is turned off (during Toff in FIG. 7A), secondary current is transiently induced into a secondary winding N2 due to counter electromotive force of the primary current accumulated in the primary winding N1.
The secondary current induced in the secondary winding N2 is charged in a smoothing capacitor 21 and so decreased linearly (FIG. 7B). At the time where the secondary current flowing through a rectifying diode 19 becomes zero, ringing phenomena appears at the primary winding N1 and feedback winding NB side.
Electric charge accumulated in the charging capacitor 14 is discharged by the ringing phenomena thereby to turn on the oscillation FET 5 again. Thereafter, the aforesaid operation is repeated to cause self-excitation between the primary side and the secondary side of the output transformer thereby to supply DC voltage to the secondary side. The self-excited RCC type switching power supply apparatus is arranged in this manner.
As described above, the repetition frequency of the self-excitation is determined by a time period (Ton period) charging the charging capacitor 14 provided at the feedback circuit and a time period (Toff period) from the start of flowing of the secondary current to the termination of flowing thereof. When a load resistor connected to an output terminal is small, electric charge accumulated in the smoothing capacitor 21 is discharged in a short time. Thus, the time period from the start of flowing of the secondary current to the termination of flowing thereof becomes long, so that the repetition frequency (oscillation frequency) of the self-excitation becomes relatively low and the output voltage also decreases. In contrast, when the load resistor is large, the secondary current is induced before the electric charge accumulated in the smoothing capacitor 21 is discharged completely. Thus, the time period from the start of flowing of the secondary current to the termination of flowing thereof becomes short, so that the oscillation frequency becomes high and the output voltage also becomes high.
A shunt regulator 24 is provided at the output terminal 27 side in order to suppress that the output voltage becomes equal to or larger than a desired voltage. The output voltage is divided by resistors 25 and 26, and the divided voltage is supplied to the control terminal of the shunt regulator 24. Since the shunt regulator 24 is operated to flow current therethrough when voltage equal to or larger than the desired voltage is applied to the control terminal thereof, current flowing into the photo diode 12b of a photo coupler 12, which is connected to the output terminal 27 through the shunt regulator 24 and a resistor 23, increases and so light emitting quantity from the photo diode becomes large. In contrast, a photo transistor 12a provided at the feedback circuit side is turned on upon receiving the light emitted from the photo diode 12b thereby to reduce an impedance between the collector and emitter terminals thereof.
Since the photo transistor 12a is connected to the charging capacitor 14 through a resistor 9 and a diode 11, the charging impedance becomes lower and so a time required for charging the capacitor becomes shorter. In this manner, since the charging time of the capacitor becomes short, the primary current decreases and hence the secondary current induced in the secondary winding side also decreases, whereby the output voltage can be suppressed to a small voltage.
As described above, although the self-excited RCC type switching power supply apparatus is able to suppress the variation of the output voltage to a small value by employing the shunt regulator 24 and the photo coupler 12 even when the output voltage varies due to the change of the load resistor, the apparatus can not suppress the oscillation frequency to a small value. In particular, when the aforesaid self-excited RCC type switching power supply apparatus is employed as a power supply for a driving circuit of a plasma display panel (hereinafter abbreviated as PDP), current becomes substantially zero during the discharge maintaining period of the PDP, and so the load current of the switching power supply apparatus does not flow thereby to increase the oscillation frequency. Accordingly, a circuit gain of the apparatus becomes large, so that the apparatus is susceptible to oscillate and the transient response thereof is degraded. In contrast, when a separately-controlled type switching power supply apparatus is employed, the size of the circuit arrangement of the apparatus becomes large and a large-sized output transformer is required.